1. Field of the Invention
The present invention relates to an intermittent magnetic recording/reproducing device. More specifically, it relates to an intermittent magnetic recording/reproducing device such as a long time video tape recorder for keeping watch mainly for preventing crimes, in which video signals are recorded intermittently on a magnetic tape.
2. Description of the Prior Art
FIG. 1 is a block diagram showing the structure of a conventional intermittent magnetic recording/reproducing device.
The structure of a conventional intermittent magnetic recording/reproducing device will be described with reference to FIG. 1. A video signal from a television camera inputted to an video input terminal is applied to a video amplifier 1 having high input impedance and is amplified. The video signal amplified by the video amplifier 1 is applied to a video recording circuit 2. The video recording circuit 2 comprises a FM modulator for modulating the video signal into a FM signal. The signal FM-modulated by the video recording circuit 2 is applied to a FM signal amplifier 3 to be amplified and applied to video heads 11 and 12 provided on a rotating drum 10 through a mode selecting switch 4. Video heads 11 and 12 record the signal on a magnetic tape. The mode selecting switch switches the operation mode between the recording mode and the reproduction mode. In the reproduction mode, a FM signal detected from the magnetic tape by the video heads 11 and 12 is applied to a video signal reproduction circuit 5. The video signal reproduction circuit 5 demodulates the FM signal detected by the video heads 11 and 12 and outputs an original video signal to a video output terminal.
An intermittent driving circuit 6 rotates a capstan shaft 16 synchronously with a vertical synchronization signal. A pinch roller 17 is provided opposed to the capstan shaft 16, whereby a magnetic tape 15 is intermittently driven by the capstan shaft 16 and the pinch roller 17. FIG. 2 is a time chart illustrating the operation of the intermittent magnetic recording/reproducing device shown in FIG. 1 and FIG. 3 is a schematic diagram showing a track of a magnetic tape on which a video signal is recorded by the intermittent magnetic recording/reproducing device shown in FIG. 1.
The operation of a conventional intermittent magnetic recording/reproducing device will be hereinafter described with reference to FIGS. 1 to 3. A video signal changed into a FM signal through the video amplifier 1, video recording circuit 2 and the FM signal amplifier 3 is applied to the video heads 11 and 12 on the rotating drum 10 through the mode selecting switch 4. The television signal transmitted from a television camera is composed of successively adjacent fields V.sub.0, V.sub.1 . . . V.sub.9, V.sub.10, V.sub.11, V.sub.12 . . . , respectively, as shown in FIG. 2(a). In a video tape recorder for intermittent recording/reproducing, if these fields are recorded continuously, much magnetic tape 15 is consumed. Therefore, an existing type video tape recorder can record at most for about 8 hours. However, if 2 fields out of every 40 fields are recorded intermittently, as shown in FIG. 2(c), recording for a long period of time, that is, 8 hours.times.40/2=160 hours becomes possible.
In order to record intermittently, the magnetic tape 15 must be moved intermittently as shown in FIG. 2(b) and video tracks which are recorded intermittently on the magnetic tape 15 must be in contact with the next track successively with inclination. Therefore, the intermittent driving circuit 6 generates an intermittent driving pulse signal on a basis of the vertical synchronization signal and applies this pulse signal to a capstan motor driving circuit (not shown). The capstan motor driving circuit accurately put forward the magnetic tape 15 intermittently by the rotation of the capstan shaft 16 and the pinch roller 17.
The aforementioned operation is completed by the writing into the magnetic tape 15 by the video heads 11 and 12 on the rotating drum 10. Similar to a common video tape recorder, the heads gap width of the video heads 11 and 12 are both as narrow as 0.25 .mu.m, and the intermittent recording is a surface layer recording in which only the surface layer of the magnetic tape 15 is used.
During reproducing, a FM signal detected by the video heads 11 and 12 from the magnetic tape 15 is applied to the video signal reproducing circuit 5 and demodulated to obtain the original video signal.
A conventional video tape recorder for intermittent recording/reproducing is structured such that the tape runs intermittently, so that an audio signal can not be recorded at the upper end of the magnetic tape (linear audio track) by a fixed head. Namely, the audio signal can not be recorded for about 0.63 second while the magnetic tape is stopped. Consequently, a conventional video tape recorder for intermittent recording/reproducing abandoned the recording of the audio signal. Accordingly, there arises a problem that due to the lack of that function, there is no audio information in the case of accident.